1. Field of the Invention
The present invention relates to a radio receiver and a radio signal processing method.
2. Related Background Art
In recent years, radio communication devices have widely spread. As a result, it is increasingly demanded to reduce the number of components and the manufacture cost for a radio circuit and manufacture the radio circuit as a monolithic IC. In order to cope with this demand, the direct conversion scheme is adopted for the radio circuits.
FIG. 8 is a block diagram of a conventional receiver adopting the direct conversion scheme. An antenna 10 receives an RF (Radio Frequency) signal, and an LNA (Low Noise Amplifier) 20 amplifies this RF signal. A quadrature demodulator 30 multiplies the amplified RF signal by an LO (Local Oscillator) signal supplied from a local oscillator (not illustrated). As a result, the RF signal is directly converted to a baseband signal. An LPF (Low Pass Filer) 40 conducts waveform shaping on the baseband signal, and a VGA (Variable Gain Amplifier) 50 amplifies this baseband signal. In addition, a demodulator 70 demodulates this baseband signal to a digital signal. In this way, the receiver using the direct conversion scheme converts the RF signal to a digital signal, and then demodulates it by using digital signal processing.
The baseband signal amplified by the VGA 50 is input not only to the demodulator 70 but also to a gain controller 60 and a DC offset canceller 94. In the gain controller 60, a signal strength detector 80 measures the strength of the baseband signal. A gain selector 90 decides whether to switch the gain of the LNA 20 and the gain of the VGA 50 on the basis of the measured value of the baseband signal. A gain control signal generator 92 outputs a gain control signal to the LNA 20 and the VGA 50 to switch the gain in accordance with the decision made by the gain selector 90. In this way, the gain controller 60 effects feedback control on the strength of the baseband signal.
The DC (Direct Current) offset canceller 94 removes a DC offset component from the baseband signal amplified by the VGA 50, and feeds back this to the VGA 50. By the way, problems concerning the DC offset component are described in “Research development tendency of mixer for direct conversion receiver (“Mission Impossible ? A Review of Mixers for Direct-Conversion Receivers”)” written by Hiroshi Tanimoto, The Transactions of the Institute of Electronics, Information, and Communication Engineers, section C, Vol. J84-C, No. 5, pp. 337–348, May 2001.
FIGS. 9A and 9B are graphs showing gains of the LNA 20 and the VGA 50, respectively. FIG. 9C is a graph showing a transient response component of a DC offset in the output of the VGA 50. The gains of the LNA 20 and the VGA 50 are simultaneously switched.
When the LNA 20 is switched from a high gain to a low gain at a point t1 in time, the VGA 50 is switched simultaneously from a low gain to a high gain. In some cases, therefore, DC offsets of both the LNA 20 and the VGA 50 overlap each other, and a very large transient response component occurs, resulting in a degraded reception performance. This is because the LNA 20 is disposed in a stage in the radio circuit preceding the VGA 50, and consequently the transient response component of the DC offset generated in the LNA 20 is amplified by the high gain obtained after the change in the VGA 50.